1. Field of the Invention
The present invention relates to a trimmer capacitor, and more particularly, it relates to a trimmer capacitor which is adapted to change an effective opposite, area of a stator electrode and a rotor electrode, which are opposed to each other through a dielectric member, by rotation of a rotor, thereby changing the capacitance.
2. Description of the Background Art
FIG. 7 shows a conventional trimmer capacitor 1, which is of interest to the present invention.
The trimmer capacitor 1 comprises a stator 2, which holds a stator terminal 3. The stator terminal 3 is integrally formed with a stator electrode 4, which is arranged to extend along the upper surface of the stator 2. The stator 2, which is formed of an insulating resin, is molded in a state receiving the integral combination of the stator terminal 3 and the stator electrode 4 therein.
A dielectric ceramic plate 5 is arranged to be in contact with a surface of the stator 2 which is provided with the stator electrode 4. A rotor electrode 6 is formed on the ceramic plate 5 to be opposed to the stator electrode 4 through the ceramic plate 5.
A center shaft 7 of a metal is arranged to pass through the stator 2 and the ceramic plate 5. The center shaft 7, comprising a driver adjusting groove 8, is electrically in contact with the rotor electrode 6. This center shaft 7 is rotated with the ceramic plate 5, to define a rotor of the trimmer capacitor 1 with the ceramic plate 5.
A rotor terminal 9 is arranged on the lower surface side of the stator 2. This rotor terminal 9 is integrally formed with a conical spring 10. The lower end of the center shaft 7, passing through the rotor terminal 9, is so crimped that the center shaft 7 and the ceramic plate 5 are jointly rotatable with respect to the stator 2 while the center shaft 7 is electrically connected with the rotor terminal 9.
In such a trimmer capacitor 1, the center shaft 7 is so rotated as to rotate the ceramic plate 5 in contact with the stator 2, thereby changing an effective opposite area of the stator electrode 4 and the rotor electrode 6. Thus, it is possible to vary the capacitance which exists across the stator terminal 3 rotor terminal 9.
However, the aforementioned trimmer capacitor 1 has the following problems:
In order to attain high maximum capacitance, it is necessary to reduce the thickness of the ceramic plate 5. However, the ceramic plate 5 may be disadvantageously cracked if the same is reduced in thickness. Although an electrode corresponding to the rotor electrode 6 may be provided in the interior of such a ceramic plate, it is technically difficult to connect such an internal electrode to the exterior and to control the effective element thickness of the ceramic plate by polishing the same, and therefore, quality is unstable in this case.
In order to build the ceramic plate 5 into the trimmer capacitor 1, on the other hand, it is necessary to sort front and rear surfaces thereof and to locate the same. Such operations are made more complicated or difficult as the trimmer capacitor 1 is miniaturized.